The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
In order to meet the ever increasing demand for improved data rate and data capacity, researchers have recently been focusing their efforts on the development of perpendicular recording systems. A traditional longitudinal recording system stores data as magnetic bits oriented longitudinally along a track in the plane of the surface of the magnetic disk. This longitudinal data bit is recorded by a fringing field that forms between a pair of magnetic poles separated by a write gap. A perpendicular recording system, on the other hand, records data as magnetic transitions oriented perpendicular to the plane of the magnetic disk. The magnetic disk has a magnetically soft underlayer covered by a thin magnetically hard top layer. The perpendicular write head has a write pole with a very small cross section and a return pole having a much larger cross section. A strong, highly concentrated magnetic field emits from the write pole in a direction perpendicular to the magnetic disk surface, magnetizing the magnetically hard top layer. The resulting magnetic flux then travels through the soft underlayer, returning to the return pole where it is sufficiently spread out and weak that it will not erase the signal recorded by the write pole.
The advent of perpendicular recording systems has lead to an increased interest in Current perpendicular to plane (CPP) sensors, which are particularly suited to use in perpendicular recording systems, due to their ability to read signals from a high coercivity medium. A CPP sensor differs from a more conventional current in plane (CIP) sensor such as that discussed above in that the sense current flows through the CPP sensor from top to bottom in a direction perpendicular to the plane of the layers making up the sensor. Whereas the more traditional CIP sensor has insulation layers separating it from the shields, the CPP sensor contacts the shields at its top and bottom surfaces, thereby using the shields as leads.
On type of CPP sensor is a tunnel valve or tunnel junction magnetoresitive sensor (TMR). Such sensors have a magnetic free layer and a magnetic pinned layer similar to a GMR or spin valve. The tunnel valve, however, has a thin electrically insulating barrier layer sandwiched between the free and pinned layers rather than an electrically conductive spacer layer. To meet the demands for increased sensor performance, researchers have sought to develop TMR sensors having improved performance characteristics. A theoretical improvement has been reported by constructing a TMR sensor having an Fe free layer, an Fe pinned layer and a MgO barrier formed therebetween. Such a construction has been proposed by Wulfhekel et al. in Applied Physics Letters, vol. 78, no. 4, 22 Jan. 2002. In reality however, TMR sensors having such a construction have not exhibited the performance that has been expected. At least a portion of this less than desirable performance has been due to the formation of Fe oxides FeOx at the interface of the free layer and the barrier layer and at the interface of the pinned layer and the barrier layer. In addition, Fe has a strong positive magnetostriction, which is undesirable for use in a free layer in that it causes magnetic instability in the sensor.
Another form of CPP sensor is a CPP GRM sensor. Such sensors have magnetic free and pinned layers and a non-magnetic, electrically conductive spacer layer sandwiched there between, similar to the more common CIP sensors. Since the current flows through the very thin GMR layers in a direction perpendicular to the plane of the layers, it can be appreciated that the overall electrical resistance, or sheet resistance (R), is extremely low. Since the performance of the sensor is the change in resistance divided by the sheet resistance, one can appreciate that as the sheet resistance approaches 0, the change in resistance also approaches 0 and the performance of the sensor drops off accordingly. Therefore, such CPP GMR sensors have not seen widespread use in commercial data storage systems.
Therefore, there is a need for a practical CPP magnetoresistive sensor having exceptional magnetoresistive performance. More particularly, there is a need for a TMR sensor having a strong dr/R performance, and a need for a CPP GMR sensor having a desired amount of electrical resistance, especially in the region of the free layer, spacer layer and pinned layer, since it is resistance in this area that contributes to GMR.